A liquid crystal display has a liquid crystal panel on which information such as images is displayed, and the liquid crystal panel has a structure in which, roughly, a liquid crystal layer is sealed between a pair of glass substrates. The pair of substrates includes an array substrate (a pixel electrode substrate, an active matrix substrate, and an element substrate) on which switching elements (such as TFT) and pixel electrodes are provided in an array shape (aligned shape), and a counter substrate disposed opposite thereto. When a voltage is applied to between a common electrode provided on the array substrate or a counter electrode provided on the counter substrate and the pixel electrode, an alignment of a liquid crystal material (liquid crystal molecules) forming the liquid crystal layer is changed to adjust an amount of light passing through the liquid crystal layer, thereby displaying images on the liquid crystal panel.
In order to control the alignment of the liquid crystal material in a voltage non-applied state, conventionally, an alignment film-forming resin including polyimide (PI) has been coated on the side of each substrate nearest to the liquid crystal layer, then a high temperature treatment (baking) has been performed to form an alignment film, and the liquid crystal material has been aligned by a memory property of the alignment film. In a structure having such a conventional alignment film, a material design of the alignment film-forming resin is optimized, and an alignment treatment employing rubbing or light irradiation is performed if necessary, whereby a degree of alignment (alignment order parameter) of the liquid crystal material in the liquid crystal layer is increased to obtain a high contrast as well as an angle of alignment can be comparatively easily controlled. However, not only are many steps necessary for forming the alignment film but also issues, such as coating irregularity and dust generation caused by the rubbing or alignment treatment, occur. To solve this, a technology which does not require the use of the conventional alignment film (hereinafter referred to as “alignment film-less liquid crystal alignment technology”) has been proposed in which a polymerizable monomer or the like is added to a liquid crystal composition and the polymerization is performed after the monomer is enclosed between substrates, thereby selectively forming a polymer layer at an interface between the liquid crystal layer and the substrate, and alignment-expressing functional groups are formed from the surface of this polymer layer, whereby the liquid crystal material is aligned.
Meanwhile, in order to meet requirements of increasing in size of a display screen, and the like, development of a horizontal alignment mode or a vertical alignment mode, configured to obtain a comparatively high contrast and a comparatively wide angle of visibility, has been advanced. Especially, it is known that according to a multi-domain vertical alignment (MVA) mode, a structure for regulating alignment is provided on a transparent electrode, whereby the alignment of a liquid crystal is changed per area of the screen, and the angle of visibility can be made wider. However, an aperture ratio may be decreased or light may leak out by the structure for regulating alignment. When the number of structures for regulating alignment is decreased in order to suppress the defects described above, then a response speed of the liquid crystal is lowered. Then, a method is proposed in which a liquid crystal composition to which a polymerizable monomer or the like has been added is enclosed between substrates, and the polymerizable monomer is polymerized in a state in which a liquid crystal material is aligned (see International Publication WO 2010/116551). This is a technology in which a polymer sustained alignment (PSA) layer is formed on a conventional alignment film to provide a pre-tilt angle (preliminary tilt angle) to the liquid crystal, whereby the delay of the response speed of the liquid crystal material is reduced while the number of structures for regulating alignment is decreased, and is referred to as “PSA technology”.
According to the alignment film-less liquid crystal alignment technology described above, the alignment treatment using the rubbing cannot be performed and it is necessary to control the alignment of the liquid crystal only by the material design of the liquid crystal composition. It is difficult, accordingly, to sufficiently align the liquid crystals, and the increase of the contrast is a problem to be solved.
On the other hand, according to the structure in which the conventional alignment films are provided on the two substrates as the conventional PSA technology, the high contrast can be expressed, but various defects caused by the formation of the alignment film cannot be avoided. The defects caused by the formation of the alignment film may include, in the array substrate, electrical connection failure and occurrence of uneven alignment, caused by difficulty in precise control of an area on which the alignment film is formed, increased moisture absorbency due to thermal denaturation of a material for forming an insulating film, and the like; and in the counter substrate, thermal decomposition and elution of a color material of a color filter layer, thermal deviation of retardation (phase difference) of an intracellular phase difference layer, and the like. All of them reduce display reliability of a liquid crystal display.